Pineal acetyl coenzyme A:arylamine-N-acetyltransferase (NAT) activity was measured after pretreatment of rats with a variety of putative transmitters and their antagonists. Pretreatment with the alpha-adrengic agents phenoxybenzamine and phentolamine prevented the rapid light-mediated decrease in NAT activity, while pretreatment with yohimbine was ineffective in altering this phenomenon. When animals were pretreated with intraventricular injections of either clonidine or norepinephrine, a decrease in pineal NAT activity was observed, mimicking the effects of light. Other adrenergic agonists were ineffective in lowering NAT activity and norepinephrine was ineffective in lowering NAT activity when administered peripherally. Phenoxybenzamine was ineffective in blocking the decrease in NAT activity observed after administration of clonidine. However, yohimbine prevented the decrease observed after pretreatment with either clonidine or norepinephrine. Thse results suggest that central alpha-receptors play a role in the modulation of NAT activity in the rat pineal.